A linear weigher has a weigh bucket mounted to a weight-sensing device with a controller that continuously records the net weight in the weigh bucket. A material feed device, such as a vibratory feeder or a gated hopper, dispenses material into the weigh bucket on command. During the filling cycle, the material from the feed device accumulates in the weigh bucket until the recorded weight equals a preset Dribble Cutoff Weight value, at which point the feed device is signaled to stop. The weigh bucket now contains an approximation of the desired weight of material. The material in the weigh bucket (dose) is then released into a container or packaging machine. The feed device is then signaled to dispense material and the weighing process is repeated.
Where the material is discrete particles of uniform individual weight, such as electrical or mechanical hardware, the weigher may be used to batch by number (count).
The linear weigher described is often referred to as a net weigher. However this invention is not limited to this type. Another type is a gross weigher where the container is placed on a weighing device and the container is filled directly by the material feeder. At the start of the weighing cycle, the empty container weight is recorded (tare weight) so that all subsequent measurements represent the weight of the material in the container. The filling cycle is identical to that described above. At the completion of the filling cycle, the filled container is moved from the weighing device and an empty container is placed for the next cycle.
To achieve higher discharge rates, multiple weighing units are mounted over a common collection funnel to direct dosed material to the container or packaging machine. The weighing units discharge sequentially into the collection funnel to achieve the desired dose rate. Typically a single controller, such as a Programmable Logic Controller (PLC) or microprocessor-based computer, controls and synchronizes all weighing units.
Most linear weighers incorporate a multi-stage feed system to increase the cycle rate by decreasing the filling time. A typical arrangement incorporates two feed devices, a bulk and dribble feed. The bulk feed dispenses material at a high rate and cuts-off at a preset Bulk Cutoff Weight value, which is less than the preset Dribble Cutoff Weight value. The dribble feed then continues until the preset Dribble Cutoff Weight is reached. Regardless of the number and configuration of the multi-stage feed system, the accuracy of the final weight in the weigh bucket is determined by the material flow characteristics of final feed stage, in this description the dribble feed.
Modern controllers can record the weigh signal with great accuracy. In fact the repeatability and accuracy from one weighing cycle to the next is determined by the constancy of the material flow rate at the dribble cut-off, and the repeatability of the feed device cut-off. These variables result in dose weights that deviate from the desired weight referred to as the Target Weight. This deviation approximates a normal curve (bell shaped), therefore the accuracy of a linear weigher is commonly specified as a weight variation at a standard deviation of 2 sigma. Ideally the average dose weight should equal the Target Weight. The Dribble Cutoff Weight is adjusted so that the average final weight in the bucket equals the Target Weight.
There are intrinsic operating conditions that cause the average dose weight to drift from the desired Target Weight. For example, there is a column of material from the outlet of the feed device to the pile of material accumulating in the weigh bucket, and any change in the weight of material in this column will affect the final dose weight. Should the bulk density of the material increase, then the weight of material in the column is higher, resulting in a dose weight greater than the desired Target Weight. This situation incurs monetary and efficiency losses due to overpack in each container. Should the material bulk density decrease then containers below the Target Weight may be produced that do not comply with consumer legislation.
The drift in the average weight is typically gradual because the underlying causes change slowly. To address this, an Automatic Weight Correction (AWC) feature has been incorporated in linear weighers for many years, which is a method to automatically adjust the weigher's operating parameters so that the average dose weight is maintained relative to the preset Target Weight. This feature minimizes changes in the average discharge weight caused by changes in product flow or density and other factors.
The AWC compensation process records the final weight after the filling process is completed, then calculates a correction value derived from the difference between the final weight and the Target Weight. This correction value is applied to the weigher's operating parameters so that the average discharge weight of subsequent weighing cycles is closer to the Target Weight.
To accurately record the final weight in the weigh bucket it is necessary to wait a period of time after the dribble feed cut-off before recording the final weight. This time allows the column of material in transit from the material feeder to be deposited in the weigh bucket and for the accumulated pile of material to stabilize. Also time is needed for the electrical and mechanical dampening, commonly used to suppress environmental vibration, to reach a steady-state weight value. This time is referred to as the Settle Time and after this delay, the weight in the weigh bucket can be recorded as the final weight. At the completion of this process, the weigher is ready to discharge the dose.
The Settle Time delay needed to record an accurate final weight for the AWC process is a substantial part of the cycle time of the weighing cycle. For example, a typical application may have the following (time in milliseconds):
Without AWCFilling time (bulk and dribble feed)3000Bucket discharge time 350Total cycle time3350Cycle rate18 doses per minuteWith AWCFilling time (from above)3350Settle Time 800Total cycle time4150Cycle rate14 doses per minute
In the above example, if the dosing application requires 54 dose per minute without AWC, then three weighing units are required. However, with AWC four weighing units are required, which adds considerable expense and complexity. For high accuracy weighing or applications that require extra weigh signal dampening (filtering), the Settle Time can be significantly more than that shown above.
It is an object of the present invention to provide a faster means to accomplish an effective Automatic Weight Control. By employing the apparatus and methods disclosed, many weigher applications can be implemented with a reduced number of weighing units resulting in substantial savings in cost and complexity. At a minimum, an existing weigher will have higher throughput.